Fuel cells are long-time known devices for the direct conversion of the chemical energy of combination of a fuel such as hydrogen and an oxidant such as air into electrical energy. Fuel cells are hence not subject to the known limitation of Carnot's cycle and are therefore characterised by a particularly high efficiency compared to the conventional devices for the production of electrical energy wherein an intermediate thermal step is present.
Among the several known types, the ion-exchange membrane fuel cell has gained a special attention for its capacity in responding to quick power demands and for the simplicity of the required auxiliaries, particularly in automotive applications and for the generation of small stationary power for domestic uses or for small communities.
The membrane fuel cell (hereafter defined in short with the acronym PEMFC from Proton Exchange Membrane Fuel Cell) consists of an electrochemical unit comprising an ionomeric membrane, either of the perfluorinated type as known in the art and as commercialised for instance by DuPont, USA under the trademark Nafion® or of the hydrocarbon type based on monomers deriving from polymeric structures such as polystyrene, polyetheretherketones and the like, on whose faces are applied two electrodes, anode and cathode, in form of porous films containing suitable catalysts (the membrane-electrode electrochemical unit is hereafter defined with the acronym CCM from Catalyst-Coated Membrane). The electrode outer surfaces are in contact in their turn with porous conductive layers, known as diffusion layers, suited to establish a homogeneous distribution of reactants, for instance hydrogen and air. The overall assembly resulting from the CCM associated to the diffusion layers (hereafter defined by the acronym MEA from Membrane-Electrode Assembly) is inserted between two planar and conductive structures provided with a higher porosity than the diffusion layers, directed to ensure both the uniform distribution of electric current and the reactant supply to the diffusion layers: such structures are known in the art as collectors. The MEA and the relevant collectors, together with suitable sealing gaskets, are finally enclosed between a pair of bipolar plates, consisting of two suitably shaped sheets, impervious to the reactants and electrically conductive. The fuel and the oxidant are supplied through openings obtained in the bipolar plates and are respectively distributed to the anode and the cathode through the collectors and the relative diffusion layers. The fuel, for example hydrogen; is oxidised with generation of protons and electrons. Protons migrate across the ionomeric membrane participating to the reduction of the oxygen contained in the air with formation of water. The electrons required for the oxygen reduction reaction come from the anode through the external electrical load circuit. The conversion efficiency of the chemical energy of reaction into electrical energy, although substantially higher than that of conventional generators, is largely below 100%: the portion of chemical energy not converted to electrical energy is dissipated as thermal energy which must be extracted by a suitable cooling device to maintain the internal PEMFC temperature typically around 60-100° C. The cooling device is preferably of forced-air type for small power systems, and of demineralised water or diathermic fluid (hereafter generally indicated as coolant) circulation type for higher power systems, which require the highest possible compactness. In the latter case, the cooling is normally effected by making the coolant flow along at least one of the bipolar plates. Since the electrical voltage of a single PEMFC under load is modest, of the order of 0.74-0.8 volts, the high voltages normally required by the load systems are obtained by assembling a multiplicity of single PEMFC intercalated to the cooling devices in blocks (hereafter stacks, as commonly known in the art).
FIG. 1 represents a longitudinal section of a possible embodiment of PEMFC stack according to the prior art disclosed in Italian Patent Application MI2003A001972, wherein (1) identifies the MEA assemblies each containing the diffusion layers, and the CCM units (7) consisting of the ionomeric membranes (2), the anodes (3) and the cathodes (4) in form of thin porous films containing the catalysts in contact with the membrane faces, the collectors (5), the bipolar plates n, peripherally sealed by gaskets (8), the cooling devices (9) each supplied with coolant and delimited by two adjacent bipolar plates (6), peripherally sealed by gaskets (10) and containing a planar conductive porous elastic element (11) which maintains the longitudinal electrical continuity. The stack furthermore comprises two plaques (12) of electrically conductive material for connecting the stack to the external electrical load circuit, each in contact with the terminal bipolar plates through a conductive element (13) equivalent to elements (11) and peripherally sealed by a gasket (14), two endplates (15) of low flexibility keeping the multiplicity of fuel cells and cooling devices under compression, ensuring a low electrical contact resistance by means of tie-rods (16) optionally provided with springs (not shown in the figure) for compensating the thermal dilatations/contractions of the various components. The tie-rods (16) are distributed in a suitable number along the perimeters of plates (15), bipolar plates (6), plaques (12) and gaskets (8), (10) and (14). The stack finally includes a sheet (17) of non conductive material for the electrical insulation of plates (15) from the stack, and connections (18), located on one of the two plates (15) to put the stack in communication with the external circuits for feeding the gaseous fuel and oxidant, for instance hydrogen and air. for withdrawing the exhausts and product water, and for the injection and extraction of the coolant. The fuel and the oxidant are respectively fed to the anodes and the cathodes by means of distributing channels obtained for example in the thickness of the gaskets (8) and connected to longitudinal manifolds fanned in the stack by lamination of suitable openings made in the different components. The injection of the coolant into the devices (9) and the discharge thereof, as well as the extraction of the mixture of exhausts and product water are carried out likewise. A critical element characterizing this and other types of stack and cooling device embodiments as described in the prior art is in the poor homogeneity of heat extraction from the fuel cell active surface, particularly in the conditions of higher electrical power output. In fact, in case of insufficient heat extraction even just from part of the active surface, the consequent temperature rise, albeit localized, causes a progressive membrane dehydration which leads to the downfall of proton conductivity with consequent worsening of performances.